or many minutes the two boats raced on at great speed over the endless waters before the watery skyline was broken far ahead by something dark and unmoving. Hackett and Norman peered with intense interest toward it. It seemed at first a giant squat mountain rising from the sea, but as they shot nearer they saw that its outline was too regular, and that colossal as it was in size it was the work of intelligence. They gasped as they came nearer and got a better view of it.
For it was a gigantic dome of black metal rising sheer from the lonely sea, ten miles if anything in diameter, a third that in greatest height. There was no gate or window or opening of any kind in it. Just the colossal, smooth black dome rearing from the watery plain. Yet the two boats were flashing lower toward it.
"They can't be going inside!" Hackett conjectured. "There's no way in and what could be in there? The whole thing's mad—"
"There's some way," Norman said. "They're slowing—"
The flying-boats were indeed slowing as they dipped lower. They were very near the dome now, its curving wall a looming, sky-high barrier before them. Suddenly the boats dipped sharply downward toward the green sea. Before the two fliers could comprehend their purpose, could do aught more than draw instinctive great breaths in preparation, the two craft had shot down into the waters and were arrowing down through the green depths.
Blinded, flung against his metal strap by the resistance of the waters they ripped through, Norman yet retained enough of consciousness to glimpse beams of light that stabbed ahead from the prows of their rushing boats, to see vaguely strange creatures of the deep blundering in and out of those beams as the boats hurtled forward. The water that forced its way between his lips was fresh, he was vaguely aware, and even as he fought to hold his breath was aware too that the frog-men seemed in no way incommoded by the sudden transition into the water, their amphibian nature allowing them to stay under it far longer than any human could do.
The boats ripped through the waters at terrific speed and in a few seconds there loomed before them the giant metal wall of the great dome, going down into the depths here. Norman glimpsed vaguely that the whole colossal dome rested on a vast pedestal-like mountain of rock that rose from the sea's floor almost to the surface. Then a great round opening in the wall; the boats flashed into it and were hurtling along a water-filled tunnel. Norman felt his lungs near bursting—when the tunnel turned sharply upward and the boats whizzed up and abruptly out of the water-tunnel into air!